Fuel vapor recovery systems are employed on motor vehicles to reduce atmospheric emissions of hydrocarbons by storing the hydrocarbons in a canister. The canister, which is coupled to the fuel tank, uses activated charcoal for absorbing the hydrocarbons. The canister is periodically purged by passing ambient air, which desorbs the hydrocarbons, through the charcoal. The resulting air and hydrocarbon mixture subsidizes the normal mixture of air, from the intake system, and fuel, from the fuel delivery system, inducted into the engine via the engine port. The canister is then able to again store hydrocarbons allowing the process to repeat.
To force ambient air through the canister, manifold vacuum is commonly used. However, manifold vacuum is a function of engine operating conditions. At certain conditions, the manifold vacuum may not be enough to force air through the canister. Also, the fuel vapor recovery process must be executed at regular intervals to assure that the canister does not become saturated. Thus, many vapor recovery systems utilize a pump, or compressor, in the system to allow purging of the canister at all operating conditions. Such a system is disclosed in U.S. Pat. No. 5,054,454.
The inventors herein have recognized numerous disadvantages with the above approaches. For example, when vapor recovery systems are utilized with direct injection engines, the requirement for purging at regular intervals is not fulfilled by simply using a pump, or compressor. In direct injection engines, the engine control system operates the engine in both a stratified mode and a homogeneous mode. In the stratified mode, a local cylinder volume containing an air and fuel mixture is surrounded by a cylinder volume of non-combustible mixture, such as pure air. In the homogenous mode, a mixture with relatively constant air/fuel properties is present throughout the cylinder volume. Conventional port injected purging systems can only form a homogeneous mixture because, when the engine inducts a mixture of air and fuel from the vapor recovery system, there is no way to form a stratified mixture. In other words, the stratified mixture depends on having an isolated region of a desired air/fuel ratio with the rest of the region filled with air. Thus, operation in a stratified mode, which is advantageous to fuel economy, is limited by the necessity to purge the canister, resulting in less than optimal fuel economy.